1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a low power driving and sensing method and system for capacitive touch panels.
2. Description of Related Art
Most of the current consumer electronics are provided with a touch pads for use as input devices. In order to meet with the light, thin, and small features, a touch pad is typically integrated with a panel as a touch panel for allowing convenient input. According to the sensing principle, the touch pad can be of resistive type, capacitive type, acoustic wave type, or optics type.
The operation principle of touch panels is to sense a voltage, a current, an acoustic wave or an infrared when a finger or other medium touches on a touch screen, so as to detect the coordinates of touching points. For example, a resistive touch panel uses the voltage difference between upper and lower electrodes to calculate the location where a force is applied, to thereby detect the touching point. A capacitive touch panel uses the current or the voltage originated from capacitance changes in a static electricity combination of transparent electrodes in row and column with human body to detect the touching coordinate.
For a capacitive touch panel, the driving is typically performed by sensing the grounded capacitance on each conductor line. Thus, a change of the grounded capacitance is used to determine whether an object is approached to the capacitive touch panel, which is known as a self capacitance sensing. Instead of being a physical capacitor, the self capacitance or the grounded capacitance is parasitic and stray capacitance on each conductor line. FIG. 1 is a schematic view of a typical self capacitance sensing. As shown in FIG. 1, during the first period of time, the driving and sensing devices 110 in a first direction drive the conductor lines in the first direction in order to charge the self capacitance of the conductor lines in the first direction. During the second period, the driving and sensing devices 110 sense the voltages on the conductor lines in the first direction. During the third period, the driving and sensing devices 120 in a second direction drive the conductor lines in the second direction in order to charge the self capacitance of the conductor lines in the second direction. During the fourth period, the driving and sensing devices 120 sense the voltages on the conductor lines in the second direction.
In the typical self capacitance sensing as shown by FIG. 1, both a driving circuit and a sensing circuit are connected to the same conductor line in order to drive the conductor line and sense a signal change on the same conductor line so as to determine a magnitude of the self capacitance. Such a self capacitance sensing has the advantages as follows:
(1) The amount of data is reduced since the typical touch panel has m+n data in a single frame only, so as to save the hardware cost;
(2) The time required for sensing a touch point is reduced since a frame row data can be quickly fetched due to only two sensing operations, i.e., concurrently or one-by-one sensing all the conductor lines in the first direction first and then in the second direction, for completing a frame, as well as a relatively reduced time required for converting a sensed signal from analog into digital; and
(3) The power consumption is relatively low due to the reduced amount of data to be processed.
However, such a self capacitance sensing may encounter the disadvantages as follows:
(1) When there is a floating conductor, such as a water drop, an oil stain, and the like, on the touch panel, it is likely to cause an erroneous decision on a touch point; and
(2) When there are multiple touch points concurrently on the touch panel, it may cause a ghost point effect, resulting in that such a self capacitance sensing cannot be used in a multi-touch application.
Another way of driving the typical capacitive touch panel is to sense a magnitude change of mutual capacitance Cm so as to determine whether the object is approached to the touch panel. Similarly, the mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor line in the first direction and the conductor line in the second direction. FIG. 2 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 2, the drivers 210 are arranged on the first direction (Y), and the sensors 220 are arranged on the second direction (X). At the upper half of the first period of time T1, the drivers 210 drive the conductor lines 230 in the first direction and use the voltage Vy_1 to charge the mutual capacitance (Cm) 250. At the lower half, all sensors 220 sense voltages (Vo_1, Vo_2, . . . , Vo_n) on the conductor lines 240 in the second direction so as to obtain n data. Accordingly, m*n data can be obtained after m driving periods.
Such a mutual capacitance sensing has the advantages as follows:
(1) It is easy to determine whether a touch is generated from a human body since a signal generated from a floating conductor is different from a grounded conductor; and
(2) Each touch point is indicated by an actual coordinate, and thus the actual position of each point can be found when multiple points are concurrently touched, so that such a mutual capacitance sensing can easily support the multi-touch application.
However, there are some disadvantages as follows:
(1) The amount of a single frame row data is m*n, which is relatively higher than the amount under the self capacitance sensing;
(2) Scanning is done by a one-to-one manner in a selected direction. For example, when there are 20 conductor lines in the first direction (Y), the sensing operation has to be performed 20 times for obtaining a complete frame row data. Also, due to the large amount of data, the time required for converting a sensed signal from analog into digital is relatively increased; and
(3) Due to the large amount of data, the power consumption is thus increased on data processing.
Therefore, it is desirable to provide an improved low power driving and sensing method and system for capacitive touch panels to mitigate and/or obviate the aforementioned problems.